Multiple party lines are well-known and are still in use in the more rural areas where economics militate against long spans of separate conductors to individual isolated subscribers.
For multiple party lines, the provision of features generally available on single party lines is generally an expensive complex process. One approach is to use subscriber multiplex apparatus for carrier signaling across a span line. This alternative is an expensive one and therefore not economically justifiable in many locations.
Other apparatus provides on a piecemeal basis certain of the features called herein single line features. For example, automatic number identification for party lines is known. One known method is to supply each handset with an oscillator, the stations of a line each generating a different frequency to be detected at the central office. Othere features have been provided, each requiring a separate package to be powered from local current in a power pack or in some cases, from the line.
Privacy is provided, for example in key systems, by a multiple relay apparatus for example, U.S. Pat. No. 3,931,478 issued Jan. 6, 1976 to B. J. Warman, and U.S. Pat No. 3,299,217 issued Jan. 17, 1976 to J. E. Burns. Such systems provide one or more features and are not readily mated with a station instrument.